Since the proliferation of modern horizontal axis wind turbines over the past decade, experience and operating statistics have demonstrated that the speed-increasing gearbox has been the source of most field failures. The most prevalent design of current wind turbines places a nacelle atop a tall tower, the nacelle housing a gearbox driven by the turbine propeller as well as the generator attached to the gearbox. To service the gearbox involves personnel entering the nacelle to perform the task of repair or replacement of the gearbox. A recent fall fatality in Great Britain underscores the danger involved in just gaining access to the top of the tower. Very large wind turbines are being deployed farther offshore at present. The operation and maintenance (O&M) costs are estimated to be twice that of their comparable terrestrial counterparts. It is therefore desirable to minimize unscheduled maintenance calls to wind turbines, to make gearboxes more reliable, and to make equipment more accessible. To achieve these objectives while also increasing efficiency is a bonus.
Bearing tooth gears are gears (in meshing gear sets) where the gear teeth are replaced by bearings thereby replacing all or most of the tooth sliding friction by rolling friction. The prior art reveals the use of plain bearings as well as rolling element bearings such as ball bearings. Many patents in the prior art relate to bearing tooth gears of a variety of types. A brief description of a few exemplary patents follows. Saladee, in U.S. Pat. No. 486,055 shows a spur type bearing gear meshing with a ring type bearing gear in a planar drive for a velocipede. Right angle drives using bearing gears (instead of bevel gears) are described in patents of Jamieson (U.S. Pat. No. 596,884) and Lloyd (U.S. Pat. No. 642,990) in relation to bicycles. U.S. Pat. No. 5,381,704 of Knotts uses bearing gears in angle drives not necessarily at right angles to each other; cylindrical bearings are used on one axis meshing with highly externally contoured bearings on the second axis to minimize backlash. Bearing tooth gears are typically used with no lubrication except for what is sealed inside each of the bearings; they reduce frictional losses thereby increasing efficiency of gearboxes or angle drives. They are inherently reliable by design.
To make equipment more accessible in a wind turbine, it is advantageous to have as much of the required equipment as possible at ground level. If a long shaft transmitting torque from the turbine atop a tower is used, the generator and gearbox can be located at ground level. This arrangement has been used to advantage especially in vertical axis wind turbines using a variety of airfoil types. Ewers (U.S. Pat. No. 4,134,707) relates to a vertical axis wind turbine using a guyed tall stack of airfoils attached to a common shaft driving a base-mounted gearbox and generator. Bolie (U.S. Pat. No. 4,204,805) illustrates a vertical axis wind turbine directly driving a long shaft within a tower down to a base-mounted vertical axis generator with integral gearbox. Japanese patent publication 2003278639 of Katsuhiro & Kazumi shows a modern three-blade horizontal axis turbine driving a long shaft through a bevel-gear angle drive which is attached to a gearbox driving a generator; both gearbox and generator are at base level and have vertical shaft orientation. U.S. Pat. No. 554,138 of Negbaur & Feely shows a multi-blade horizontal axis wind turbine driving a long vertical shaft via a bevel gear angle drive. The bottom of the long shaft has a second bevel gear angle drive to power a direct drive horizontal axis generator at ground level.
The prior art does not reveal modifications of bearing tooth gear systems for wind turbine use nor their integration with large horizontal axis wind turbines to improve reliability, accessibility, and efficiency.